Retroviruses can be divided into several classes which include lentiviruses, such as HIV, and oncoviruses, such as the murine leukemia viruses (MuLV). HIV in humans and a unique isolate of MuLV in mice induce profound immunodeficiency and late developing lymphomas although the mechanisms are likely quite different in the two species. We study the disease in mice, termed MAIDS, for mouse AIDS, which is caused by a MuLV that cannot replicate on its own and encodes only an unusual Gag polyprotein as the sole viral determinant required for disease. Our overall goal is to understand how this infection influences the biology of lymphocytes to cause immune defects and cancer. These investigations involve in vivo analyses of different mouse strains for their sensitivity to disease, studies of cultured cell lines obtained from infected mice, molecular genetic assays of the causative virus and studies of cytokines in relation to disease.